This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. Section 119 from an application for xe2x80x9cMethod and System for Locating Mobile Station in Mobile Telecommunication Systemxe2x80x9d filed in the Korean Industrial Property Office on Mar. 11, 1999 and there duly assigned Serial No. 1999-8023.
1. Field of the Invention
The present invention relates generally to a cellular mobile telecommunication network, and in particular, to a method and system for locating a mobile station (MS) in a CDMA (Code Division Multiple Access) cellular network.
2. Description of the Related Art
The goal of a cellular mobile telecommunication network is to allow any registered subscriber to make a call to anyone at any place and time. FIG. 1 illustrates a typical mobile telecommunication network configuration. Referring to FIG. 1, the mobile communication network is comprised of a plurality of base stations (BSs) 21 to 24, a mobile station (MS) 10, a base station controller (BSC) 30 for controlling the BSs 21 to 24, and a mobile switching center (MSC) 50 for connecting the BSC 30 to another BSC or a public switched telephone network (PSTN).
In the thus-constituted cellular mobile telecommunication network, the whole service area is divided into a plurality of coverage areas of BSs. Each BS coverage area is called a cell. The MSC 50 controls the BSs 21 to 24 so that a subscriber can continue his or her call connection without interruption while moving between cells.
The MSC 50 can reduce the time required for processing a call by accurately locating the cell where the subscriber is located. In case of an emergency, such as fire or a situation where a patient needs first aid treatment, the mobile subscriber should be accurately located. Tracking the location of a mobile subscriber within a cell in a mobile telecommunication network is known as location service.
The MS can be located within the boundary of a given cell by the inter-working function of the MS or by the mobile telecommunication network.
In the former case, the MS is provided with a GPS (Global Positioning System) receiver to calculate its location in latitude and longitude coordinates based on the location information received from the satellite through the GPS receiver. The requirement of the extra GPS receiver, however, increases the manufacturing cost and the size of the MS.
As an alternative to the self-locating method using the satellite, the MS implements a signal receiver to calculate its location by trigonometry, which is based on the signals received from at least three BSs. This method also increases the cost and size of the MS due to the requirement of a separately procured signal receiver. Moreover, the MS can not operate compatibly with some BSs using a different form of signal.
As stated above, the self-locating method increases the load on the MS as it is required to calculate its location. As a result, errors might occur in the calling process of the MS.
In the case that the network is involved to locate the MS, at least three BSs receive a signal from the MS. The network calculates the distance between the three BSs and the MS using the arrival time of the signal at the BSs, then determines the location of the MS using the trigonometry. This location service is provided generally by a location data processor 40 included in the BSC or is provided independently. Upon request for a location service about a specific mobile subscriber, i.e., MS by a user, the BSC 30 selects the BSs 21, 22, and 23 for use in the location service in order to locate the MS. The network can calculate the location of the MS using the time of arrival (TOA) or the time difference of arrival (TDOA).
The TOA method calculates the distance between the MS and the BS based on the TOA of a signal received at each BS from the MS. It is assumed that the MS is located at the intersection point of three circles whose respective radius is the respective distance between each BS and the MS.
The TDOA method assumes that the TDOAs of a signal transmitted from the MS at each of the three BSs define a set of points on a hyperbola and the MS is located at the intersection point of at least three hyperbolas. The implementation of this method requires accurate synchronization of each BS, as compared to the TOA method.
As described above, the network tracks the location of an MS using a specific signal transmitted from the MS. However, the MS signal is often propagated to a BS in a longer path than the actual distance between the MS and the BS due to the multipath fading and the NLOS (Non-line-of-sight) effect in a real mobile telecommunication environment. In this case, at least three circles or hyperbolas do not meet at one point but overlap an area, and the location area of the MS is estimated to be too large. Therefore, the location data processor 40 should detect the most likely point where the mobile telephone is located in the overlap area. However, if the estimated MS location area is too wide, the location data processor has difficulty determining the accurate location of the MS. That is, it is highly probable that errors occur in estimating the location of the MS when the selected estimated location area is too large.
The conventional location estimating method, therefore, can cause serious errors in estimating the location of an MS because the location area of the MS is estimated to be too large due to multipath fading and propagation delay.
It is, therefore, an object of the present invention to provide a mobile station (MS) locating method and system in which an MS location area is estimated using the time of arrival (TOA) method and then narrowing the MS location area using the inventive Angle of Arrival (AOA) system, in order to more accurately determine the location of an MS.
To achieve the above object, there is provided a method and system for locating a mobile station, and an AOA determining system. According to one aspect of the present invention, the MS is capable of communicating with at least three BSs, each having at least two antennas. To estimate the location of the MS, the distance between the MS and each BS is calculated based on the time of arrival (TOA) of an MS signal at the BS, a first estimated MS location area is defined as an overlap area of circles whose respective radius is the respective distance between the MS and the three BSs. Then, the location of the MS is estimated based on the time of signals received through two antennas provided in each BS, and then the angle of arrivals of the signal with respect to the antennas of each BS and the MS is utilized. Here, the angle is calculated from the phase difference between the received signals and the communication frequency of the signals. A second location area is defined as an area defined by the, lines derived from AOA operation, connecting the BSs to the MS. Then, the MS is determined to be located in the common area of the first and the second location areas.
In a system for estimating the location of an MS in a CDMA mobile communication system according to another aspect of the present invention, each of at least three BSs is capable of communicating with the MS and includes two different antennas for calculating the TOA, the TDOA, and the phase differences between the antennas. The system calculates a coarse AOA and an estimated AOA between the MS and the antennas based on the TOA, the TDOA, and the phase difference and determines a final AOA between the MS and the antennas. The MS is capable of communicating with the three BSs. A location data processor estimates first and second MS location areas from an MS signal received at the BSs, and determines a common area of the first and the second MS location areas as the real location of the MS.